MOUNT_NFS(8)

NAME
mount_nfs - mount NFS file systems

SYNOPSIS
mount_nfs [-23KLPTUbcdils] [-I readdirsize] [-R retrycnt] [-a maxreadahead] [-g maxgroups] [-m realm] [-o options] [-r readsize] [-t timeout] [-w writesize] [-x retrans] rhost:path node

DESCRIPTION
The mount_nfs command calls the mount(2) system call to prepare and graft a remote NFS file system (rhost:path) on to the file system tree at the point node. This command is normally executed by mount(8). It implements the mount protocol as described in RFC 1094, Appendix A and Appendix I.

By default, mount_nfs keeps retrying until the mount succeeds. This behaviour is intended for file systems listed in fstab(5) that are critical to the boot process. For non-critical file systems, the -b and -R flags provide mechanisms to prevent the boot process from hanging if the server is unavailable.

If the server becomes unresponsive while an NFS file system is mounted, any new or outstanding file operations on that file system will hang uninterruptibly until the server comes back (or that NFS file system is forcibly unmounted). To modify this default behaviour, see the -i and -s flags.

The options are:

-2  Use the NFS Version 2 protocol.

-3  Use the NFS Version 3 protocol. The default is to try version 3 first, and fall back to version 2 if the mount fails.

-I readdirsize
    Set the readdir read size to the specified value. The value should normally be a multiple of DIRBLKSIZ that is <= the read size for the mount.

-K  Pass Kerberos authenticators to the server for client-to-server user-credential mapping. This requires that the kernel be built with the NFSKERB option. (Refer to the INTERNET-DRAFT titled for more information.)

-L  Do not support NFS file locking operations. Any attempt to perform file locking operations on this mount will return the error ENOTSUP regardless of whether or not the NFS server supports NFS file locking.

-P  Use a reserved socket port number. This is useful for mounting servers that require clients to use a reserved port number on the mistaken belief that this makes NFS more secure. (For the rare case where the client has a trusted root account but untrustworthy users and the network cables are in secure areas this does help, but for normal desktop clients this does not apply.)

-R retrycnt
    Set the retry count for doing the mount to the specified value. The default is 10000.

-T  Use TCP transport instead of UDP. This is recommended for servers that are not on the same LAN cable as the client.

-U  Force the mount protocol to use UDP transport, even for TCP NFS mounts. (Necessary for some old BSD servers.)

-a maxreadahead
    Set the read-ahead count to the specified value. The default is 4. This may be in the range of 0 - 16, and determines how many blocks will be read ahead when a large file is being read sequentially. Trying larger values for this is suggested for mounts with a large bandwidth * delay product.

-b  If an initial attempt to contact the server fails, fork off a child to keep trying the mount in the background. Useful for fstab(5), where the file system mount is not critical to multiuser operation.

-c  For UDP mount points, do not do a connect(2). This must be used for servers that do not reply to requests from the standard NFS port number 2049. It may also be required for servers with more than one IP address if replies come from an address other than the one specified in the requests.

-d  Turn off the dynamic retransmit timeout estimator. This may be useful for UDP mounts that exhibit high retry rates, since it is possible that the dynamically estimated timeout interval is too short.

-g maxgroups
    Set the maximum size of the group list for the credentials to the specified value. This should be used for mounts on old servers that cannot handle a group list size of 16, as specified in RFC 1057. Try 8, if users in a lot of groups cannot get a response from the mount point.

-i  Make the mount interruptible, which implies that file system calls that are delayed due to an unresponsive server will fail with EINTR when a termination signal is posted for the process.

-l  Used with NFSv3 to specify that the ReaddirPlus RPC should be used. This option reduces RPC traffic for cases such as “ls -l”, but tends to flood the attribute and name caches with prefetched entries. Try this option and see whether performance improves or degrades. Probably most useful for client to server network interconnects with a large bandwidth times delay product.

-m realm
    Set the Kerberos realm to the string argument. Used with the -K option for mounts to other realms. This requires that the kernel be built with the NFSKERB option.

-o  Options are specified with a -o flag followed by a comma separated string of options. See the mount(8) man page for possible options and their meanings. The following NFS-specific options are also available:

    bg  The same as -b. Retry mount in background.

    noconn
        The same as -c. Do not connect UDP sockets.

    dumbtimer
        The same as -d. Turn off the dynamic retransmit timeout estimator.

    intr
        The same as -i. Make the mount interruptible.

    nfsv2
        The same as -2. Use the NFS Version 2 protocol.

    nfsv3
        The same as -3. Use the NFS Version 3 protocol.

    nfsvers=<num>

    vers=<num>
        Set the NFS protocol version number - 2 for NFSv2 and 3 for NFSv3.

    rdirplus
        The same as -l. Use the ReaddirPlus RPC with NFSv3

    resvport
        The same as -P. Use a reserved socket port number.

    soft
        The same as -s. Make the mount soft. Fail file system calls after a number of retries.

    udp Use UDP transport. This is currently the default.

    tcp The same as -T. Use TCP transport instead of UDP.

    mntudp
        The same as -U. Force the mount protocol to use UDP transport, even for TCP NFS mounts.

    acregmin=<seconds>

    acregmax=<seconds>

    acdirmin=<seconds>

    acdirmax=<seconds>
        These options set the minimum and maximum attribute cache timeouts for directories and "regular" (non-directory) files. The default minimum is 5 seconds and the default maximum is 60 seconds. Setting both the mininum and maximum to zero will disable attribute caching. The algorithm to calculate the timeout is based on the age of the file or directory. The older it is, the longer the attribute cache is considered valid, subject to the limits above.

    actimeo=<seconds>
        Set all minimum and maximum attribute cache timeouts to the same value.

    noac
        Disable attribute caching. This is equivalent to setting actimeo to 0.

    nolocks

    nolockd

    nolock

    nonlm
        The same as -L. Do not support NFS file locking operations.

    rsize=<readsize>
        The same as -r. Set the read data size to the specified value.

    wsize=<writesize>
        The same as -w. Set the write data size to the specified value.

    rwsize=<size>
        Set both the read data size and write data size to the specified value.

    readahead=<maxreadahead>
        The same as -a. Set the maximum read-ahead count to the specified value.

    timeo=<timeout>
        The same as -t. Set the initial retransmit timeout.

    retrans=<count>
        The same as -x. Set the retransmit timeout count for soft mounts.

-r readsize
    Set the read data size to the specified value. The default is 8192 for UDP mounts and 16384 for TCP mounts. It should normally be a power of 2 greater than or equal to 1024. Values greater than 4096 should be multiples of 4096. It may need to be lowered for UDP mounts when the “fragments dropped due to timeout” value is getting large while actively using a mount point. (Use netstat(1) with the -s option to see what the “fragments dropped due to timeout” value is.) See the -w option as well.

-s  A soft mount, which implies that file system calls will fail after Retry round trip timeout intervals.

-t timeout
    Set the initial retransmit timeout to the specified value. May be useful for fine tuning UDP mounts over internetworks with high packet loss rates or an overloaded server. Try increasing the interval if nfsstat(1) shows high retransmit rates while the file system is active or reducing the value if there is a low retransmit rate but long response delay observed. (Normally, the -d option should be specified when using this option to manually tune the timeout interval.)

-w writesize
    Set the write data size to the specified value. Ditto the comments w.r.t. the -r option, but using the “fragments dropped due to timeout” value on the server instead of the client. Note that both the -r and -w options should only be used as a last ditch effort at improving performance when mounting servers that do not support TCP mounts.

-x retrans
    Set the retransmit timeout count for soft mounts to the specified value.

EXAMPLES
The simplest way to invoke mount_nfs is with a command like:

    mount remotehost:/filesystem /localmountpoint
or:
    mount -t nfs remotehost:/filesystem /localmountpoint

It is also possible to automatically mount file systems at boot from your /etc/fstab by using a line like:

    remotehost:/home /home nfs rw 0 0

PERFORMANCE
As can be derived from the comments accompanying the options, performance tuning of NFS can be a non-trivial task. Here are some common points to watch:

•   Increasing the read and write size with the -r and -w options respectively will increase throughput if the network interface can handle the larger packet sizes.

    The default read and write sizes are 8K when using UDP, and 16K when using TCP. Values over 16K are only supported for TCP, where 60K is the maximum.

    Any value over 32K is unlikely to get you more performance, unless you have a very fast network.

•   If the network interface cannot handle larger packet sizes or a long train of back to back packets, you may see low performance figures or even temporary hangups during NFS activity.

    This can especially happen with lossy network connections (e.g. wireless networks) which can lead to a lot of dropped packets.

    In this case, decreasing the read and write size, using TCP, or a combination of both will usually lead to better throughput.

•   For connections that are not on the same LAN, and/or may experience packet loss, using TCP is strongly recommended.

ERRORS
Some common problems with mount_nfs can be difficult for first time users to understand.

    mount_nfs: can't access /foo: Permission denied

This message means that the remote host is either not exporting the file system you requested or is not exporting it to your host. If you believe the remote host is indeed exporting a file system to you, make sure the exports(5) file is exporting the proper directories. The program showmount(8) can be used to see a server's exports list. The command “showmount -e remotehostname” will display what file systems the remote host is exporting.

A common mistake is that mountd(8) will not export a file system with the -alldirs option, unless it is a mount point on the exporting host. It is not possible to remotely mount a subdirectory of an exported mount, unless it is exported with the -alldirs option.

The following error:

    NFS Portmap: RPC: Program not registered

means that the remote host is not running mountd(8). The program rpcinfo(8) can be used to determine if the remote host is running nfsd, and mountd by issuing the command:

    rpcinfo -p remotehostname

If the remote host is running nfsd, mountd, rpc.statd, and rpc.lockd it would display:

    program vers proto   port
     100000    2   tcp    111  portmapper
     100000    2   udp    111  portmapper
     100005    1   udp    950  mountd
     100005    3   udp    950  mountd
     100005    1   tcp    884  mountd
     100005    3   tcp    884  mountd
     100003    2   udp   2049  nfs
     100003    3   udp   2049  nfs
     100003    2   tcp   2049  nfs
     100003    3   tcp   2049  nfs
     100024    1   udp    644  status
     100024    1   tcp    918  status
     100021    0   udp    630  nlockmgr
     100021    1   udp    630  nlockmgr
     100021    3   udp    630  nlockmgr
     100021    4   udp    630  nlockmgr
     100021    0   tcp    917  nlockmgr
     100021    1   tcp    917  nlockmgr
     100021    3   tcp    917  nlockmgr
     100021    4   tcp    917  nlockmgr

The error:

    mount_nfs: can't get net id for host

indicates that mount_nfs cannot resolve the name of the remote host.

SEE ALSO
mount(2), unmount(2), fstab(5), mount(8,) umount(8,) nfsstat(1), netstat(1), rpcinfo(8), showmount(8), automount(8)

CAVEATS
An NFS server shouldn't loopback-mount its own exported file systems because it's fundamentally prone to deadlock.